1. Field of the Invention
The present invention relates to polymer blends. More particularly, the present invention relates to blends of copolymers of alkyl(meth)acrylates with radically polymerizable carboxylic acid anhydrides and vinyl aromatics, and copolymers of styrene and/or .alpha.-methyl styrene and acrylonitrile or methacrylonitrile.
2. Description of the Prior Art
The material polymethylmethacrylate (PMMA) is a thermoplastic which exhibits valuable properties, such as transparency and high weather resistance, good processibility, etc. However, PMMA also exhibits weaknesses such as a relatively high tendency to absorb water and an impact strength that does not meet all requirements. Water absorption of PMMA, under saturation conditions (23.degree. C.), is 2 wt. %. To reduce the water absorption, copolymerization with hydrophobic monomers such as vinyl aromatics, presents itself first. In European Published Patent Application No. 0 268 040, compatible polymer blends are described, for example, that comprise cyclohexyl(meth)acrylate-containing acrylate copolymers and .alpha.-methyl styrene-containing polymers with or without moieties of acrylonitrile or maleic acid anhydride, which are characterized by their high transparency.
Of course, copolymerization of vinyl aromatics does not guarantee a decrease in water absorption in every case. Measurements show that in the case of copolymers comprising methyl methacrylate (MMA), styrene and maleic acid anhydride in the ratio of 75:15:10 parts by weight (commercial product Plexiglas.RTM. HW55) the saturation value of water absorption is 4 wt. %. Water absorption of this magnitude can have a negative impact during repeated, relatively extreme, dry-moist cycles, as can occur with outdoor weathering. Molded parts made of these polymers show environmental stress cracking.
For a long time the idea of improving the impact strength of thermoplastics like PMMA by mixing with elastomeric polymers has also been pursued. The requirement that the material obtained be transparent has had a limiting effect on the options for blending. Polymer blends comprising PMMA, styrene-acrylonitrile copolymers in proportion by weight (78-88):(22-12) and acrylonitrile-butadiene-styrene copolymerizates (ABS) are known from German Offenlegungsschrift 28 28 517 and 20 45 742. Instead of PMMA, copolymers of methyl methacrylate with alkyl methacrylate can also be added as mix components.
Other thermoplastic moulding compositions are known, e.g., from German Offenlegungsschrift 36 01 423. They contain 25-60 wt. % of polycarbonate, 5-40 wt. % of polyalkylene terephthalate, 2-30 wt. % of grafted rubber and copolymer comprising 10-90 wt. % of (methyl)styrene with 90-10 wt. % of (methyl)acrylic-acid, -ester or -nitrile, maleic acid-anhydride or -imide and/or glycidyl methacrylate.
Another possibility represents blends comprising 1-99 wt. % of a copolymer with the components methyl methacrylate, maleic acid anhydride and an aromatic vinyl monomer and 99-1 wt. % of a copolymer comprising 80-100 wt. % of methyl methacrylate with.0-20 wt. % of an ethylenic monomer (European Published Patent Application No. 0 113 105).
The study by R. A. Mendelson [J. Polym. Sci., Polym. Phys. Ed 23 (10) 1975-1995 (1985)] gives a reference to compatible mixing systems based on styrene-acrylonitrile (SAN) copolymers; as a consequence of which the miscibility or the incompatibility of maleic acid anhydride-MMA-styrene copolymers with SAN copolymers is dependent on the relative maleic acid anhydride content, on the one hand, and the acrylonitrile content, on the other hand. However, only compatible mixing systems comprising styrene-MMA-maleic acid-anhydride copolymers with a styrene content above 50 wt. % and styrene-acrylonitrile copolymers with a styrene content between 90 and 40 wt. % can be derived from this study so that the assumption that compatibility is based on the high proportion of styrene of the components seemed justified.
The desire for materials than can be processed like a thermoplastic and combine excellent mechanical properties with very good optical properties can be satisfied only to a limited degree with the options offered by the prior art. The materials should combine, as far as possible,
--high flow during processing
--high impact strength and notched impact strength
--transparency or a minimum of haze and
--low water absorption
without forfeiting, on the other hand, the desired qualities of the parent components. Thus, any possibility that allowed the use of known and commercially available copolymers would be especially interesting.